1. Field of the Invention
The present invention relates to an organic light emitting display and, more particularly, to a full color active matrix organic light emitting display that can reduce a step height between an organic thin film layer and a lower electrode and relieve a taper angle of a substrate surface to prevent device failure from occurring.
2. Description of the Related Art
Generally, an active matrix organic light emitting display (AMOLED) has a structure such that a thin film transistor (TFT) is arranged on a substrate in a form of a matrix, an anode connected to the TFT is formed, and an organic thin film layer and a cathode are formed thereon.
FIG. 1 is a cross-sectional view of a conventional bottom-emission organic light emitting display. Referring to FIG. 1, a buffer layer 105 is formed on an insulating substrate 100, and a semiconductor layer 110 including source/drain regions 111, 115 is formed on the buffer layer 105. A gate electrode 125 is formed on a gate insulating layer 120, and source/drain electrodes 141, 145 each connected to the source/drain regions 111, 115 through first holes 131, 135 are formed in an interlayer insulating layer 130.
An anode 170, a lower electrode connected to the drain electrode 145 of the source/drain electrode 141, 145 through a second hole 155, is formed on a passivation layer 150 and the drain electrode portion at the second hole 155, and an organic thin film layer 185 and a cathode 190, which is an upper electrode, are formed thereon.
In the conventional organic light emitting diode having the above structure, when a taper angle of the first hole or the second hole is large, there occurred a pinhole defect at the stepped region between around the second hole or the first hole and the anode 170, and a short defect of the anode and the cathode. Further, since the organic emission layer is not uniformly deposited around the first hole and the second hole and at the stepped region of the anode, there were problems that a dark spot was generated by a concentrated phenomenon of current density when a voltage between the anode and the cathode was applied, and that an emission region was reduced due to the generation of the dark spot to degrade picture quality.
To address the foregoing problems, an organic light emitting display employing a pixel defining layer made of the organic insulating layer with a planarization feature was disclosed in U.S. Pat. No. 6,246,179.
FIG. 2 is a cross-sectional view of a conventional organic light emitting diode having a pixel defining layer. Referring to FIG. 2, a buffer layer 205 is formed on an insulating substrate 200, and a semiconductor layer 210 including source/drain regions 211, 215 are formed on the buffer layer 205. A gate electrode 225 is formed on a gate insulating layer 220, and source/drain electrodes 241, 245 connected to the source/drain regions 211, 215 through first holes 231, 235 are formed in an interlayer insulating layer 230.
An anode 270, which is a lower electrode connected to one of the source/drain electrodes 241, 245, e.g. the drain electrode 245, through a second hole 255, is formed on a passivation layer 250 and the drain electrode portion at the second hole 255. A pixel defining layer 265 including an opening 275 that exposes a portion of the anode 270 is formed, and an organic thin film layer (i.e., organic emission layer) 285 and a cathode 290, which is an upper electrode, are formed on the exposed portion of the anode 270 and the pixel defining layer 265. The organic thin film layer 285 has at least one organic emission layer such as a hole injection layer, a hole transporting layer, an emitting layer of R, G or B, a hole blocking layer, an electron transports layer and an electron injection layer.
The conventional top-emission organic light emitting display as described above solved the problem of the device defect caused by the stepped region of the substrate surface by using the pixel defining layer 265. However, when the organic emission layer is formed by the laser induced thermal imaging process, device reliability is changed depending on the taper angle and the step between the pixel defining layer 265 and the anode 270.
FIGS. 3A to 3C are cross-sectional views for illustrating a method of forming an organic emission layer using the laser induced thermal imaging process.
Referring to FIG. 3A, a TFT including the semiconductor layer 210, the gate electrode 225 and the source/drain electrodes 241, 245 is formed on the insulating substrate 200, and the anode 270 connected to the drain electrode 245 of the source/drain electrodes 241, 245 through the second hole 255 arranged in the passivation layer 250 is formed, as shown in FIG. 2. The pixel defining layer 265 having an opening 275 to expose a portion of the anode 270 is formed. A donor film 20 having an organic emission layer 21 is aligned and closely adheres to the substrate on which the TFT is formed (TFT substrate).
Next, as shown in FIG. 3B, while the donor film 20 adheres to the TFT substrate, a laser is irradiated with a pattern onto a predetermined region. When the laser is irradiated on the donor film 20, a portion of film where the laser is irradiated is expanded and the organic emission layer 21 of the donor film 20 is patterned on the insulating substrate 200. When the donor film 20 is removed from the TFT substrate after completing a transfer process, an organic emission layer pattern 285a is formed on the anode 270 and the sidewall of the pixel defining layer 265, as shown in FIG. 3C.
When the organic emission layer pattern 285a is formed by the laser induced thermal imaging process, a distance h31 from the surface of the organic emission layer 21 to the upper surface of the anode 270 has a close relationship to the energy required during the laser induced thermal imaging process. That is, when the pixel defining layer 265 is deposited thick so that there is a high step between the pixel defining layer 265 and the anode 270, the distance h31 to which the donor film 20 should reach the opening 275 should be relatively increased. Further, to this end, the donor film 20 should be expanded relatively large, so that the irradiation energy of the laser should be increased.
As the irradiation energy becomes large, the surface temperature of the donor film 20 is also increased to the extent more than required, so that the characteristics of the organic emission layer pattern 285a transferred to the anode 270 and the pixel defining layer 265 are changed. When the characteristics of the emission layer are changed, there were problems of characteristic deterioration, such as the deterioration of the efficiency of the resultant organic light emitting display, a change in the color coordinate, and reduction of the life-time.
Further, when the organic emission layer is formed by the laser induced thermal imaging process, the donor film and the TFT substrate should adhere tightly with each other. However, when the taper angle of the pixel defining layer 265 is large, the donor film does not adhere tightly to the insulating substrate at the edge, so that there occurs an open defect 285c, or the organic emission layer pattern 285a within the opening 275 is open. Particularly, the donor film does not adhere tightly to the TFT substrate at the edge of the opening 275, so that the open defect 285c is generated at the edge of the opening 275. That is, when the organic emission layer is not rightly transferred or even when it is regularly transferred, a defect such as unclean transfer boundary is generated.
FIG. 6 is a picture showing an edge open defect of the conventional organic light emitting display having a pixel defining layer with a large taper angle and a high step. Referring to FIG. 6, it shows that when the pixel defining layer is formed at a taper angle of more than 40° and in a step height of 10,000 Å, the edge open defect is generated at the opening, which is a boundary between the anode and the pixel defining layer. During the laser transfer process, the characteristics of the organic emission layer are changed by the high energy required due to the high step of the pixel defining layer, so that the efficiency of the organic emission layer is degraded to less than 30%. In this case, for the blue organic emission layer, the color coordinate was changed from 0.15, 0.18 to 0.17, 0.25 along with the efficiency reduction, and for the red organic emission layer, a mixed color phenomenon occurs due to the emission of the electron transporting layer at the portion where the edge open defect is generated.